RALEIGH — A one-day flu vaccination blitz for health care workers that began at Duke University Heath System last year is now an annual event, spreading to more than a dozen clinics and hospitals across the Triangle.

The 24-hour mass vaccination for the coming flu season begins Wednesday morning with two goals: To inoculate as many of the more than 15,000 health care workers as possible, and to practice the regional response to a pandemic. Emergency response teams simulated deliveries of vaccine Tuesday from the state’s strategic stockpile.